cvs commit: src/sys/dev/md md.c

Pawel Jakub Dawidek pjd at
Thu Sep 16 15:30:30 PDT 2004

On Thu, Sep 16, 2004 at 03:15:15PM -0700, Nate Lawson wrote:
+> You should be checking the work condition in thread 2 while holding the 
+> mutex but before going to sleep.  Adding work to the queue happens in 
+> thread 1 where you write "..." and that is done with the mutex held so 
+> there is no race.  The full diagram with this detail included is:
+> thread1               thread2
+> -----------------------------
+> mtx_lock(mtx)
+> add work to queue
+> mtx_unlock(mtx)
+>                       mtx_lock(mtx)
+> wakeup(ptr)
+>                       check queue for work item
+>                       if (!work item)
+>                           msleep(ptr, mtx)
+>                       else
+>                           dequeue work item and loop
+> Since the work item is added in thread1 with the mutex held, the check 
+> for it in thread2 is safe and race-free.  A wakeup is only there to 
+> kickstart thread2 if it's asleep.  If it's running, it needs to check 
+> atomically that there is no work before sleeping.  If it doesn't do 
+> this, it's a bug.

Yes, you are right.

Pawel Jakub Dawidek             
pjd at                 
FreeBSD committer                         Am I Evil? Yes, I Am!
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